explicit arrays and limit checks

[Matthew Fluet]

> the native codegen.  Matthew, can you take a look?  The main addition
> to the native codegen is the translation of Machine RuntimeOperands
> into x86 operands.  Maybe that messes up some liveness info?

I took a quick look and didn't see anything obvious; I fixed a bug in the
definition of gcState_limitPlusSlop and checked the other gcState
variables.

With a program:
val _ = print "Hello World!\n"
the segfault is occuring because we're trying to allocate an object when
frontier (or, at least what is in %edi) is greater than gcState.limit.
This is occuring after allocating a non-pointer array, so the array is
actually reaching into this area, but we're not initializing it yet.

I'll try to take another look later today.